The present invention relates to high dry content polyurethane compositions based on perfluoropolyether oligomers. Said compositions are used for application to various substrata, such as for example metals, ceramic and stony materials, plastic and glass surfaces and they are able to completely crosslink also at temperatures lower than 20xc2x0 C., also of about 10xc2x0 C., forming thin films having good mechanical and surface properties.
Coatings obtained from polyurethane formulations based on perfluoropolyethers (PFPE) are known in the prior art. According to EP 665,253 it is possible to prepare high dry content polyurethane resins, by crosslinking PFPEs having hydroxyl terminations and functionality comprised between 2 and 4, or mixtures thereof, with polyisocyanic adducts obtained by reaction of polyols with diisocyanates, of the type for example TMP-IPDI (trimethylolpropane-isophorondiisocyanate) having a ratio by moles 1/3). Said formulations have the drawback not to completely crosslink at a temperature of 25xc2x0 C. or lower. Therefore the properties of the obtained coating are unsatisfactory as regards the chemical resistance, permeability and duration in the time.
From EP 812,891 bicomponent xe2x80x9cno dirt pick upxe2x80x9d formulations are known in which the PFPE resin diol is crosslinkable with prepolymers obtained by prepolymerization of a PFPE diol with polyisocyanates obtained by cyclotrimerization of HDI, IPDI or other monomeric diisocyanates, alone or in admixture therewith. The examples of this application describe formulations having a dry content higher than 70% by weight but not higher than 85% by weight. The xe2x80x9cno dirt pick upxe2x80x9d properties are good but it is desirable to further improve them. In conclusion, the protective coatings obtainable by crosslinking the prior art formulations, containing PFPEs having hydroxyl terminations, with polyisocyanates, have a lower crosslinking temperature limit, generally from +25xc2x0 C., which compromises the possibility of application and complete crosslinking in the field and they do not show an optimal combination of properties as indicated hereinafter.
The need was therefore felt to have available PFPE-based polyurethane compositions combining the following properties:
applicability to dry contents greater than 80% by weight, preferably in the range 85-90% by weight,
filming capability and complete crosslinkability also at temperatures in the range 5xc2x0 C.-20xc2x0 C.,
as regards the properties of the coatings obtainable by said formulations:
improved chemical resistance and stability to UV ageing photooxidation,
improved xe2x80x9cno dirt pick usxe2x80x9d surface properties,
good mechanical properties and in particular good surface hardness (pencil hardnessxe2x89xa7B) and abrasion resistance.
Compositions based on polyisocyanates and perfluoropolyethers meeting the combination of the above mentioned properties have now been surprisingly and unexpectedly found by the Applicant.
An object of the present invention are compositions for coatings having a dry content higher than 80%, preferably of about 85-90% by weight, based on polyisocyanates and PFPE, completely crosslinkable also in a range of temperatures between 5xc2x0 C. and 20xc2x0 C., said compositions comprising the following components:
Component 1): mixture comprising:
1.a) Partially fluorinated prepolymers, having free NCO groups, obtained by reaction of (per)fluoropolyethers (PFPEs) diols having number average molecular weight Mn in the range 800-1,500, preferably 1,000-1,200, with the cyclic trimer of the isophorondiisocyanate (IPDI), in said reaction the ratio in equivalents between the OH/NCO groups being in the range 0.20-0.25,
1.b) non cyclic isocyanic trimer of hexamethylendiisocyanate (biuret of HDI) having an absolute viscosity at 20xc2x0 C. lower than 5,000 mPa.s, in component 1) the ratio referred to the dry product between the compound 1.b) and the compound 1.a) being in the range 10-90, preferably 30-60 parts of compound 1.b)/100 parts of compound 1.a);
Component 2): (per) fluoropolyether (PFPE) diol having Mn in the range 350-700, preferably 500-0650, the amount of PFPE diol component 2) being such that the ratio in equivalents between the OH and NCO groups in the composition is in the range 0.9-1.1;
Component 3): inert organic solvent under the crosslinking conditions, complement to 100% by weight of the composition.
The component 1.a) is obtainable by hot dissolving (for example 40xc2x0-80xc2x0 C.) the trimer of IPDI and the (per)fluoropolyether diol in a solvent as those indicated in component 3), maintaining the stoichiometric ratio in equivalents OH/NCO within the indicated limits and a dry content in the range 65%-85% by weight. Then the polymerization catalyst is added, see later on, preferably dibutyltin-bis-isooctylthioacetate and the reaction is hot maintained until the theoretic NCO content is obtained, determined by titration according to ASTM D 2572.
As component 2 also (2a) mixtures of PFPE oligomer diols having Mn in the range 800-1,500, preferably 1,000-1,200, with PFPE oligomer diols having Mn in the range 350-700, preferably 500-650, can be used, in said mixtures of oligomers the weight ratio between, respectively, the high and low molecular weight oligomers being in the range 1/2-1/10, or alternatively the number average molecular weight of the mixtures of PFPE oligomer diols being lower than or equal to 700.
The inert solvent component 3) under crosslinking conditions is a solvent which does not react with the OH and NCO groups present in the composition of the invention.
As component 3) an aprotic dipolar solvent such as ethyl or butyl acetate is preferably used.
As cyclic trimer of isophorondiisocyanate (IPDI) of component 1.a, the commercial product Vestanat(copyright) T1890, Huels can for example be used.
As component 1.b) non cyclic isocyanic trimer of hexamethylendiisocyanate (biuret of HDI), the commercial products Tolonate(copyright) HDB-LV, Rhodia(copyright) or Desmodur(copyright) 3200, Bayer, can be used.
The (per)fluoropolyether diol compounds comprise one or more of the following (per)fluorooxyalkylene units xe2x80x94(C3F6O)xe2x80x94, xe2x80x94(CFYO)xe2x80x94, xe2x80x94(C2F4O)xe2x80x94, xe2x80x94CR4R5CF2CF2Oxe2x80x94,xe2x80x94(CF2)axe2x80x2Oxe2x80x94, wherein Y is F or CF3, R4 and R5 are equal to or different from each other and selected from H, Cl, axe2x80x2 is an integer equal to 3 or 4.
The preferred PFPE diol compounds for the compositions of the present invention are the following, wherein the (per) fluoropolyoxyalkylene units are statistically distributed along the chain:
aI) xe2x80x94(C3F6O)mxe2x80x2 (CFYO)nxe2x80x2xe2x80x94 wherein the (C3F6O) and (CFYO) units are perfluorooxyalkylene units statistically distributed along the chain; mxe2x80x2 and nxe2x80x2 are integers such as to give the above mentioned molecular weights, and mxe2x80x2/nxe2x80x2 are comprised between 5 and 40, nxe2x80x2 being different from 0; Y is F or CF3;
when the unit (CFYO) is absent in this case nxe2x80x2 can also be equal to 0;
bI) xe2x80x94(C2F4O)pxe2x80x2(CYFO) qxe2x80x2xe2x80x94(C3F6O)txe2x80x2xe2x80x94 wherein pxe2x80x2 and qxe2x80x2 are integers such that pxe2x80x2/qxe2x80x2 ranges between 5 and 0.3, preferably between 2.7 and 0.5 and such that the molecular weight is within the above indicated limits; txe2x80x2 is an integer with the meaning of mxe2x80x2, Y=F or CF3; txe2x80x2 can be 0 and qxe2x80x2/(qxe2x80x2+pxe2x80x2+txe2x80x2) is equal to 1/10or lower and the txe2x80x2/pxe2x80x2 ratio ranges from 0.2 to 6;
cI) xe2x80x94CR4R5CF2CF2Oxe2x80x94 wherein R4 and R5 are equal to or different from each other and selected from H, Cl, the molecular weight such as to be comprised in the above mentioned limits, a fluorine atom of the perfluoromethylene unit can be substituted with H, Cl, or perfluoroalkyl group, having for example from 1 to 4 carbon atoms;
dI) xe2x80x94(CF2)axe2x80x2Oxe2x80x94 wherein axe2x80x2 is an integer equal to 3 or 4. The (per) fluoropolyethers are obtainable by known processes. See U.S. Pat. Nos. 3,665,041, 3,242,218, 3,715,378 and EP 239,123.
The two end groups, equal to or different from each other, of the bifunctional (per)fluoropolyethers are of the type
HO (CH2CH2O)x0CH2xe2x80x94CF2xe2x80x94
wherein x0 is an integer from 0 to 4, preferably from 0 to 2; in the preferred compounds x0=0; said end group being linked to the (per)fluorooxyalkylene unit by an oxygen atom but not by peroxidic sequences xe2x80x94Oxe2x80x94Oxe2x80x94.
The formulations according to the present invention advantageously combine the following properties:
complete crosslinkability at temperatures in the range +5xc2x0 C.-+20xc2x0 C.;
very high dry content when applied, also higher than 85%; therefore the formulations of the present invention are able to meet the most restrictive rules in terms of VOC (volatile organic compound) emissions and therefore they can be applied also in crowded urban places;
improved properties of xe2x80x9cno dirt pick upxe2x80x9d, waterproofing, chemical resistance and abrasion-resistance;
high surface hardness (pencil hardnessxe2x89xa7B);
high gloss and transparency;
good mechanical properties such as for example high hardness and flexibility.
It has been surprisingly found that the biuret of HDI component 1.b) as above defined, at the indicated crosslinking temperatures forms homogeneous mixtures both with the partially fluorinated prepolymers having free NCO groups component 1.a) and with the PFPEs diols component 2) as above defined. Other commercial crosslinking agents as the isocyanurate trimers, have not this property, as described in the comparative Examples.
Furthermore, the number average molecular weight of component 2) comprised in the above indicated limits allows to obtain an almost instantaneous homogeneous mixture with the free polyisocyanates of component 1.b) (not prepolymerized).
The compatibility among said components makes it possible the preparation of formulations having a high titre of dry product and reduced viscosity as requird for the application in the field.
The use of PFPE diol mixtures component 2), having bimodal distributions of the molecular weight allows to further improve the above indicated properties and in particular the xe2x80x9cno dirt pick upxe2x80x9d properties.
The compositions according to the present invention can be formulated both as monocomponent and as bicomponent, therefore they can be obtained by mixing the components 1.a), 1.b) and 2) with the solvent, or the same components are suitably combined to give the bicomponent formulation A (components 1.a+1.b) and B (component 2), the solvent being added in A and/or in B.
The obtained formulations are limpid and macroscopically transparent.
The compositions of the invention are crosslinked by addition of a catalyst (only at the time of the use). The chemical classes of catalysts are well known, the metal or amine ones commonly used in the paint formulations can be mentioned, for example: tertiary amines such as triethylendiamine, N-ethyl-ethylendiamine, tetramethylguanidine, dimethyl cyclohexylamine, diazobicyclo octane, etc.; organometal catalysts such as dibutyltindilaurate, tin octanoate, cobalt naphthenate, vanadium acetylacetonate, dimethyl-tin-diethylhexanoate, dibutyltin diacetate, dibutyltin dichlodride, and mixtures thereof. The catalyst is added in concentrations generally ranging from 0.1 to 2% by weight and preferably from 0.5 to 1%.
To the formulations of the invention additives such as for example pigments and fillers, preferably dispersed or predispersed in component 2), can be added. When silica having an opacifying action of the coating are used, they must be incorporated in a minimum amount in order to avoid worsenings (brittleness) of the mechanical properties. Pigments can be selected from the natural, synthetic inorganic and organic ones. As an example of inorganic pigments it can be mentioned: metal oxides (titaniun dioxide, iron oxides, mixed oxides of Ni, Co, Zn, Ti, or Cr, Cu or Fe, Ni, Cr, Mn, cobalt aluminates); organic pigments (derivatives from anthraquinone, quinacridone, tetrachloroisoindolinone, diketoperylene, phthalocyanines, etc.). Fillers as well as inert pigments and any other type of additive, are added to the dispersion under a very fine size, generally with sizes lower than 100 microns, preferably lower than 10 xcexcm, in amounts up to 15% by volume. Other examples of compounds usable as fillers are: polyamides having 20-100 microns sizes, glass spheres such as zeolites, etc.
Other optional additives are thixotropic agents, polymer dispersing agents acrylic, silicone, polyurethane, polyamine or having a carboxylic or non ionic functionality; stretching, anticissing, antifoam additives, etc.; additives to reduce photooxidation (UV adsorber for example hydroxybenzophenone derivatives, hydroxybenzotriazole, etc.) and HALS (hindered amines, for example derivatives from tetramethyl-piperidine, etc.).
The pot life, defined as the necessary time to double the initial viscosity of the formulation, for monocomponent formulations maintains high both at 20xc2x0 C. and at 50xc2x0 C., in a closed vessel in absence of the crosslinking catalyst. The pot life is generally of at least 4 hours at 50xc2x0 C. and at least 8 hours at 20xc2x0 C.
By adding to the formulation small amounts (10-50 ppm) of a suppressor of the catalyst used for the synthesis of the prepolymer component 1.a), for example benzoyl chloride, phosphoric acid or Irganox(copyright) MD, the shelf life of the monocomponent formulation is of various days at room temperature.
The application tachniques are the conventional ones for protective coatings, such as for example spray-gun, brush, roll. In the preferred application technique by spray-gun the suitable viscosities for the formulations are in the range 100-500 mPa.s.
Besides in the case of monocomponent formulations the viscosity values at 50xc2x0 C. are sufficiently low also with the highest dry product contents, therefore allowing a correct application using the above indicated techniques.
The formulation pot-life is of about 10 minutes from the time of the addition of the crosslinking catalyst and therefore when said formulations are spray applied, it is preferable to use guns equipped with a system of mixing heads, optionally heatable, heads known in the prior art and commercially available, wherein the addition of the catalyst at the gun head at the spray-time, with optional recycle of the not sprayed formulation, is involved.
The coatings obtainable by crosslinking at room temperature of the formulations according to the present invention are homogeneous and transparent films, having a 5-100 microns, preferably 30-50 microns, thickness.
The following Examples describe the invention with non limitative purposes.